This is a revised request from the University of California at San Diego for funds to purchase an advanced electron paramagnetic resonance (EPR) spectrometer. A User's Group has been established at UCSD involving 8 faculty and staff from 4 UCSD campus departments and 1 staff at The Scripps Research Institute; 8 NIH funded PI's are represented. The individual research activities to be supported by the purchase of the spectrometer include studies of metal binding sites in heme proteins and proteins of respiration and photosynthesis, models for metalloprotein clusters and metal quinone complexes, conformational changes in enzymes subject to complex regulatory signals, assembly of molecular aggregates on or within lipid bilayers, interactions of lipids with membrane proteins and the photochemical conversion of Green Fluroescent Protein to a form that emits red light. The spectrometer of choice is the Bruker ELEXSYS system, operating at X-band frequency. Justifications for the choice of this instrument include its state-of-the-art capabilities as a continuous wave spectrometer and its capacity to be expanded (with future additional funding) to carry out more advanced EPR experiments, such as multifrequency EPR, ENDOR and ESEEM. We anticipate a future need for such experiments. Funds are also requested for the purchase of appropriate cryogenic apparatus, a TM110 mode cavity for more sensitive recording of protein bound spin-labels and an NMR gaussmeter to obtain accurate g-values. The total cost of the system is 390,571 dollars. An Administrative Arrangement and a Long-Term Operation Plan are presented.